Field of the Invention
The present invention relates to an isolation transformer to be used under a high voltage, and an X-ray generating apparatus and a radiography system each including the isolation transformer.
Description of the Related Art
In general, an X-ray generating apparatus includes an X-ray generating tube configured to generate an X-ray by irradiating a target with an electron beam flux emitted from an electron gun, a tube voltage generating device configured to apply a high voltage between an anode and a cathode of the X-ray generating tube, and a drive device for the electron gun. Further, there has been known a mono-tank X-ray generating apparatus in which those respective members are disposed in a container. The mono-tank X-ray generating apparatus may be applied to a portable X-ray generating apparatus and is advantageous in size reduction.
Meanwhile, the drive device for the electron gun includes an isolation transformer configured to transform a voltage of a drive signal from a power source located outside of the X-ray generating apparatus into a cathode potential reference. A primary side of the isolation transformer is close to a ground potential and a secondary side thereof substantially has a cathode potential. Thus, the isolation transformer is required to have a high breakdown voltage.
In order to reduce the X-ray generating apparatus in size, the isolation transformer is required to be reduced in size. As one solution for this, there is an isolation transformer using a toroidal core. In Japanese Patent Application Laid-Open No. H11-74135, as a technology for providing a high-voltage isolation transformer using a toroidal core, there is disclosed a structure in which a core is covered by a resin case and coils are wound around the core and the resin case, thereby increasing breakdown voltages of the core and the resin case.
An insulating liquid is generally filled into the X-ray generating apparatus in order to ensure an internal breakdown voltage and cool the X-ray generating tube. The X-ray generating apparatus is filled with an insulating liquid as follows: the X-ray generating tube and other necessary devices are housed in a container, and the container is then evacuated. When the isolation transformer of Japanese Patent Application Laid-Open No. H11-74135 is applied to such an X-ray generating apparatus, gas bubbles may be trapped in the resin case due to an insulating liquid permeating into the resin case during the insulating liquid filling. In general, as the insulating liquid, a mineral oil that has a higher dielectric constant than gas bubbles (air) is used. Thus, if gas bubbles are trapped in the resin case of the isolation transformer, an electric field tends to be concentrated on the gas bubbles, resulting in a reduction in breakdown voltages of portions in which the gas bubbles remain. As a result, reliability of the apparatus is reduced in terms of driving of the electron gun.